We too need to be able to have an event with sub-events and limit the sign-up number per sub-event. We have an outing with different hikes available, but each hike needs a registration cap. Once the registration cap for the particular hike is met, the system should not allow people to sign up for that hike.

Plus, if you sign up for an all day hike, you should not be able to accidentally sign up for one of the 1/2 day hikes for the same day.